A carefully managed cycle

The growing trend of operating combined-cycle plants in varying modes places stresses on the HRSG

Heat recovery steam generators are increasingly operating in cycling mode according to market demands. This requires a holistic, top-down and integrated management of assets to maximise the overall profitability of a combined-cycle plant, writes Alstom’s Pascal Decoussemaeker

Driven by the demands of today’s deregulated markets and the influx of intermittent renewable generation into the grid, combined-cycle power plants increasingly have to respond to large load fluctuations in a short time.

To meet the challenges associated with the various modes of operation, the heat recovery steam generator (HRSG) within a combined-cycle plant must be capable of frequent starts, rapid load transients and prolonged periods of operation at low loads for spinning reserve.

At the same time, HRSGs have become more complex with the rapid development of gas turbine technology. As HRSG pressures and temperatures have increased, units have become larger and new designs have been introduced to accommodate the changing steam parameters.

Although many of these HRSGs are still relatively new, it is important to manage the condition of this asset from the outset in order to maintain continued safe operation and maximise operating lifetime.

Pro-active analysis and monitoring of assets can provide valuable insight into weak links or constraints in the design that can limit response rates or hamper the ability to continuously operate according to a particular operation profile.

The information provided by such analysis and monitoring can be used to establish limits for plant operation and assess the financial impact. If the impacts are severe, then options to modify the design can be explored in a cost-benefit analysis. This approach allows a balance to be struck between risk to the reliability and availability of the HRSG, and the potential financial returns from having a more flexible plant.

Asset management

Asset management was introduced to determine the optimum and most efficient use of a plant’s assets. Essentially, owners and operators have shifted their focus from simple maintenance and cost reduction to the optimisation of overall plant competitiveness. Items such as operating strategy and production losses are now all part of the equation.

For effective asset management it is important to find the optimum compromise between risk taking and risk mitigation. This is only possible if there is sufficient knowledge of the condition of the HSRG’s parts.

The planning of maintenance and inspection works should also reflect the criticality of the equipment. It should aim to optimise the allocation of scarce resources – such as people, specialised equipment and spare parts – to maintain the lifecycle value of the asset and not compromise production or EHS (environmental protection, health management and safety) commitments.

Acquiring the correct knowledge of the asset’s condition requires a good understanding of the most critical areas and to implement a plan to monitor the asset’s condition so it is maintained in accordance with the initial design goal. This is possible with the right combination of off-line inspections and on-line monitoring.

Asset degradation

A good understanding of a plant’s history and future operation profile are important to make an analysis of the main drivers of asset condition degradation. The thermal and mechanical flexibility of a HRSG is heavily dependent on the fundamental layout and detailed design of its components.

Creep will be a degradation driver for components that are exposed for long periods to high temperatures, such as superheater and reheater tubes, outlet headers, manifolds and piping that typically operate at temperatures above 500 à‚ºC.

In situations where plants are increasingly cycled, however, fatigue damage of certain pressure parts and structural parts, such as hot casings, can occur. Fatigue damage usually results from thermal stresses due to temperature differences within, or between, parts. For example, thick-walled components, such as drums or superheat outlet headers, can develop significant through-wall temperature gradients during startup. Large temperature differences can also occur at junctions between thick- and thin-walled parts, such as tube-to-header connections.

Through-wall temperature gradients in thick-walled components can cause fatigue cracking on the internal (water/steam heated) surface, particularly around penetrations where so-called ‘star-burst’ cracking – radial cracks emanating from the edge of the hole – is common. Detection of this type of damage requires, as a minimum, internal (borescopic) inspection, in combination with ultrasonic inspection.

Temperature differences between parts, such as tube-to-header connections, can result in cracking from either the internal or external surface, depending on the nature of the transient temperature history. External surface cracks generally form at the toe of the weld in the thinner wall of the components that are joined, such as the tube of a tube-to header connection. Internal cracks are less common, but can occur at the root of full-penetration tube-to-header welds.

These fatigue cracks that are the result of cycling are generally sharp and straight. The cracks may be oxide-filled – held open or shut by residual stresses – so careful inspection is often required to detect them. Dye penetrant or magnetic particle testing is typically used to find these cracks. Advanced knowledge of where to employ these techniques based on an understanding of operating history and damage mechanism greatly facilitates their application to the appropriate locations.

Cycling-related failures

Other cycling-related failure modes can be triggered by poor operational practices, inappropriate control logic or defective valves.

A common case is damage to components downstream of desuperheaters caused by improper spray control logic. High steam temperatures at very low steam flows during start up and shut down may trigger desuperheater spray while there is insufficient steam flow to entrain the spray flow. This results in water pooling in the bottom of pipes and large top-to-bottom temperature differences, which cause the pipe to bow and generate potentially damaging local stresses in the adjacent components. Leaking spray valves can also cause the same problem.

Low load operation is becoming increasingly popular as a way of avoiding constant start up and shut downs of the gas turbine. This usually results in higher gas turbine exhaust temperatures and lower exhaust gas flows, which change the balance of heat pick-up in the HRSG, often resulting in a bias toward higher heat pick-up in the finishing superheater.

This, in turn, requires higher amounts of desuperheater spray to maintain acceptable steam temperatures into the steam turbine. The higher amounts of spray flow combined with reduced steam flow results in longer distances required downstream of the desuperheater for complete spray evaporation. This can result in spray impingement on downstream components generating high thermal stresses.

To mitigate these effects it is important to first understand the various operating scenarios and spraywater requirements. There must also be a balance between local metal temperatures – higher temperatures might be permissible at lower operating pressures – and spray capacity. In certain circumstances, adjustments to heat pick-up or desuperheater or piping design may be needed to optimise operation over the full range of operating scenarios.

The reduced steam flows at low load also mean reduced water velocities in economisers. If velocities are sufficiently low, buoyancy forces can cause reverse flow in some tubes of down-flow economiser banks. This generates tube-to-tube temperature differences, resulting in thermal stress.

In many cases the issues mentioned cannot be detected or diagnosed using the standard plant control (DCS) instrumentation. Invariably, additional local thermocouples, to directly detect overspray for example, and heat balance calculations are needed.

The examples outlined occur frequently in practice and have been known to cause damage affecting the functionality of components in a relatively short period of time. This highlights the need to continuously review operating scenarios and practices in relation to the original design goal and past operation. Changes in modes of operation should be reviewed to establish their impact on the asset to ensure future reliability.

Based on this assessment, it is important to develop an effective condition monitoring system to get an early warning of potential issues. This can be achieved by combining on-line monitoring and off-line inspections.

On-line monitoring

Inspection and monitoring quantify the progress of the degradation and provide assurance that the asset integrity is maintained in accordance with the design. There are two different types of monitoring during operation: monitoring the symptoms of a certain problem and monitoring key process parameters that trigger the degradation.

For HRSGs, monitoring the symptoms is limited to visual and other non-destructive inspections during the operation of the plant. This can be used to identify failure modes that lead to, for example, steam leaks, exhaust gas leaks, external corrosion and internal wall loss.

Key process parameters are monitored using a HRSG life-monitoring tool to check elements that contribute to important failure modes. This tool allows operators to monitor the accumulation of creep and fatigue damage for critical pressure parts.

Key temperature, pressure and steam flows from the plant instrumentation are monitored and calculations are made to determine temperature profiles and stresses within pressure parts. The variation of stress with time is monitored by the system so cycles can be counted to determine fatigue damage, and operating time at a certain temperature can be used to determine creep damage.

Such a system will provide reports and displays to allow operators to monitor damage over time. It also provides an indication of conditions that need immediate attention to avoid severe damage.

For such a tool to be effective, it is important that the calculation algorithms have all been validated against appropriate benchmark cases and are compliant with prevailing codes and standards. Material data used by the system must be based on published values. This can be assured if the tool has been certified by an independent agency, such as for example TUV.

Off-line inspections

Periodic off-line inspections are required to review the asset condition for degradation modes that cannot be inspected on line, or to further investigate areas identified during on-line monitoring. For HRSGs, this needs to be planned alongside the gas turbine inspection to optimise overall plant availability.

Priorities for inspection should be established based on a combination of a review of the degradation modes, as well as the types of operation and the potential effects on the HRSG components. This invariably requires a review of the HRSG design to understand high-stress locations. Knowledge of the operational history is also required, particularly for transient scenarios and off-design conditions.

For large fleets, experience can also provide valuable feedback. Standardised designs facilitate inspection planning as components requiring specific inspection procedures will be consistent unit to unit. It should be noted, however, that HRSGs of the same design that operate behind different gas turbines may experience different issues as the turbines generate different thermal transients in the HRSGs.

To do the up-front review and properly identify concerns requires a thorough consideration of numerous failure modes and degradation phenomena. This calls for a systematic approach to catalogue information and perform calculations to screen components based on their susceptibility to relevant failure mechanisms.

Software that supports this type of review over the lifetime of the equipment is very useful. Such a tool should allow the cataloguing of unit information, such as drawings and previous inspection results, performing relevant calculations to assess risk to components based on operating conditions and past history. It should also support the management of engineering reviews and recommendations.

Internal inspections can also provide insight to operational issues. Damage to some internal components may be indicative of the need to further evaluate other areas of the HRSG. Damage to drum separators through flow accelerated corrosion, for example, may be an indication that water chemistry is not correct and that the corrosion is occurring elsewhere in the evaporator section.

In addition to the functional components carrying the steam and water and the structural components that support the HRSG, it is important to assess items such as baffles and insulation. Damaged gas baffles can allow gas bypassing, which can reduce performance. This can also result in localised high temperatures on downstream components that could cause more severe fatigue because of steaming and tube-to-tube temperature differences in economisers. Loss of insulation in wall and roof panels has resulted in damage to casing and, in some cases, structural steel.

No upfront screening calculations can identify such local effects, which highlights the need for a co-ordinated effort based on both engineering review and competent inspection. This allows plant operators to move from a reactive mode to a pro-active one, which pays off in the long run.

Holistic approach

Effective management of HRSG assets to maximise the overall profitability of a combined-cycle plant requires a holistic approach. The operational requirements of the plant must be considered, based on business needs, asset condition and the expected drivers of asset degradation, according to component design, as well as past, current and anticipated future operational conditions.

A holistic approach is key to the effective management of HRSG assets

Once such a review has identified the areas with the highest criticality, this analysis must then be used to determine an effective condition monitoring programme.

Pascal Decoussemaeker is product manager, HRSG at Alstom. For more information, visit www.alstompower.com

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